1. Field of the Invention
The invention is directed to a computed tomography apparatus of the type having with a gantry that is rotatable around a rotational axis and means for determining an unbalanced mass of the gantry.
2. Description of the Prior Art
A computed tomography apparatus of the above type is disclosed, for example, in U.S. Pat. No. 5,610,968. The computed tomography apparatus has a gantry mounted to be rotatable around a rotational axis and at which the components such as, for example, an X-ray source or a radiation detector that rotate around a patient under examination together with the gantry during operation of the gantry, are arranged. If the gantry has an unbalanced mass, i.e., a tumbling motion of the gantry radially and/or axially relative to its rotational axis, can lead to unsharp images produced with the computed tomography apparatus. The unbalanced mass of the gantry is therefore determined with a specific imbalance indicator during manufacture of the computed tomography apparatus, and the gantry is subsequently provided with one or more weights in order to compensate the unbalanced mass or at least reduce it within a suitable range of tolerance. Suitable imbalance indicators are offered, for example, by the Brxc3xcel and Kjaer Vibro company (Brxc3xcel and Kjaer Vibro GmbH, Landwehrstrasse 55, 64293 Darmstadt).
After a replacement of the components arranged at the gantry, a renewed balancing of the gantry is usually required. To that end, the balancing device must be brought to the location at which the computed tomography apparatus is located.
Such a balancing device, however, is relatively expensive and the operation thereof is relatively complicated, so that a renewed balancing after a replacement of one of the components arranged at the gantry can be relatively costly.
It is therefore an object of the invention to provide a computed tomography apparatus which allows for a simpler balancing of the gantry.
According to the invention, the object is achieved in a computed tomography apparatus having a gantry that is rotatable around a rotational axis, an imbalance detector for determining an unbalanced mass of the gantry, and a calculation unit for calculating the location or locations of the gantry at which a weight or weights should be arranged for compensating the unbalanced mass. The computed tomography apparatus thus has components integrated into the apparatus for determining an unbalanced mass of the gantry. As a result, no external balancing device needs to be brought to the location of the computed tomography apparatus after a replacement of components at the gantry. A complicated adjustment of the external balancing device is thus eliminated, so that errors in the balancing can be reduced and even a less skilled technician can reliably balance the gantry. Moreover, the detector integrated in the computed tomography apparatus for determining an unbalanced mass can be specifically adapted to the computed tomography apparatus in order to enable a more precise balancing. During maintenance, a technician normally does not have an external balancing device available for cost reasons. A further advantage of the inventive computed tomography apparatus is therefore that the technician can economically and dependably check the unbalanced mass of the gantry as well during the course of maintaining the computed tomography apparatus.
After the unbalanced mass of the gantry is determined with the detector, the calculating unit calculates the location or locations at the gantry where the weight or the weights should be arranged for compensating the unbalanced mass. These locations can, for example, be made visible with a monitor of a control computer of the computed tomography apparatus. A relatively simple identification of the locations for arranging the weights is thus achieved.
In a version of the invention, the detector for determining an unbalanced mass is a measurement sensor that acquires the vibration radially or axially relative to the rotational axis of the gantry. This collaborates with a position acquisition unit which detects the position of the gantry relative to a stationary part of the computed tomography apparatus that is likewise part of the detector for determining an unbalanced mass. When the measurement sensor acquires vibrations radially relative to the rotational axis, conclusions about the unbalanced mass of the gantry radially relative to the rotational axis can be made in a known way, on the basis of a linkage of the vibration radially relative to the rotational axis and the position of the gantry relative to the housing of the computed tomography apparatus. The acquisition of the vibration axially relative to the rotational axis in conjunction with the acquisition of the position of the gantry relative to the housing of the computed tomography apparatus, in contrast, allow conclusions to be made about an unbalanced mass of the gantry axially relative to the rotational axis.
In a preferred embodiment of the invention, the detector for determining an unbalanced mass include at least two measurement sensors that acquire the vibration radially and axially relative to the rotational axis of the gantry. Then the unbalanced mass of the gantry radially as well as axially relative to the rotational axis of the gantry can be determined. The measurement sensors can be arranged at the computed tomography apparatus so that, for example, one measurement sensor only acquires the vibration radially relative to the rotational axis and the other measurement sensor only acquires the vibration axially relative to the rotational axis.
Knowledge of the position of the gantry relative to the stationary housing of the computed tomography apparatus during the rotation of the gantry is necessary for the reconstruction of images produced with the apparatus. In a preferred embodiment of the invention therefore the detector for determining an unbalanced mass that includes a detector necessary for the acquisition of the position of the gantry relative to a stationary part of the computed tomography apparatus for the reconstruction of images produced with the computed tomography apparatus. The detector for position acquisition of the gantry relative to a stationary part of the computed tomography apparatus need be employed only once. As a result costs can be saved in the manufacture of the computed tomography apparatus.
In a further embodiment of the invention, the detector for determining an unbalanced mass of the gantry includes a computer that can be connected to an information transmission network, so that data representing an identified unbalanced mass of the gantry can be communicated to a central data bank. The central data bank, for example, can be operated by the manufacturer, a sales organization or a distribution organization for the computed tomography apparatus. It can also be operated by a service vendor who maintains the computed tomography apparatus. It is thus possible for the detector for determining an unbalanced mass of the gantry to check the unbalanced mass of the gantry automatically, for example every time when the computed tomography apparatus is in operation, in order to recognize a change in the unbalanced mass of the gantry. If, for example, the unbalanced mass of the gantry lies outside a prescribed tolerance range the computer can automatically contact the central data bank via the information transmission network. It is also possible for the computer to retain data that represent an unbalanced mass of the gantry and that lie beyond the range of tolerance in a memory of the computer, so that the manufacturer, the distribution organization, the sales organization or the service vendor regularly contacts the computer of the computed tomography apparatus via the information transmission network and uses the data bank to read out the memory of the computer. In both instances, thus, the manufacturer, the distribution organization, the sales organization or the service vendor is quickly informed about the modified unbalanced mass of the gantry, and accordingly can quickly dispatch a technician to the computed tomography apparatus in order to re-balance the gantry. The unbalanced mass of the gantry of a computed tomography apparatus that is operating is thus essentially continuously monitored.
In another version of the invention, the calculating unit for calculating the location or locations of the gantry is a control computer of the computed tomography apparatus. Beyond calculating those locations at which the weights should be arranged for balancing the gantry, the control computer controls the rotation of the gantry, the x-ray source and the radiation detector during the production of images of a patient with the computed tomography apparatus.
In a preferred embodiment of the invention, the calculating unit for calculating the location or locations of the gantry take locations into consideration at which components are arranged at the gantry. These components, for example, according to one version of the invention, are an x-ray source, a radiation detector and/or a cooling device for cooling an x-ray tube of the x-ray source. Obviously, no weights or balancing the gantry can be arranged at the locations at which components are already arranged. In known balancing devices, this fact is ignored and locations for arranging the weights are determined which may interfere with components already arranged on the gantry. In order to nonetheless balance the gantry, an experienced technician must find those locations on the basis of the locations determined by the balancing device at which there is still space for the weights. The precision of the balancing is thus dependent on the experience of the technician implementing the balancing. Even such a technician, who, however, must make a number of balancing attempts until the gantry is balanced within the prescribed tolerance range.
Since, inventively, the calculating unit for calculating the location or locations of weights takes into consideration the locations at which components are already arranged at the gantry, i.e. only determine those locations for the arrangement of the weights at the gantry where there is still space, the balancing can be quickly and relatively precisely implemented by a less experienced technician. The need for a number of balancing attempts is also eliminated, so that time and thus costs can be reduced.